It is generally known that process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various consumer products. Each of these process control loops is designed to keep some important process variable such as pressure, flow, level, or temperature, within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates load disturbances that affect the process variable and control of the process control loops within the plant. To reduce the effect of these load disturbances, the process variables are detected by sensors or transmitters and communicated to a process controller. The process controller processes this information and provides changes or modifications to the process loop to get the process variable back to where it should be after the load disturbance occurs. The modifications typically occur by changing flow through some type of final control element such as a control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or a chemical compound, to compensate for the load disturbance and maintain the regulated process variable as close as possible to the desired control or set point.
It is generally understood that various control valve configurations may be specifically applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. Alternatively, when precise control over a large control range is required, a sliding stem control valve may be used. In any configuration, such control valves are generally coupled to a control device such as an actuator, which controls the exact opening amount of the control valve in response to a control signal. Thus, when designing a process, the process engineer must consider many design requirements and design constraints. For example, the design engineer must determine the style of valve used, the size of the valve, the type of actuator, etc.
In some systems, especially in pneumatically controlled fluid process systems, the actuator for any given fluid process control device may include a diaphragm actuator. Typical diaphragm actuators comprise a housing containing a spring-biased diaphragm assembly. The diaphragm assembly is operatively coupled to a flow control element via a valve stem or other actuator rod, in order to control the opening amount of the fluid process control device.
Some operating systems require precise control of fluid flow characteristics throughout the entire operating range of the control valve. Diaphragm control valves are used for such precise control. One type of diaphragm fluid control valve includes a cage inserted within a fluid flow corridor. A control diaphragm assembly, including at least a control diaphragm, moves relative to one end of the cage, which acts as a seat, to control fluid flow through the valve. Such control diaphragm control valves are particularly advantageous for systems in which fluid flow characteristics need to precisely controlled throughout the entire operating range of the valve. Control diaphragm control valves also provide more reliable sealing in a closed position, particularly at low flow conditions.
Different types of control diaphragms may be used in the control valve based upon system requirements or characteristics. For example, high temperature systems may require a metal diaphragm to resist the high temperatures without failure. On the other hand, lower temperature systems may allow the use of an elastomeric diaphragm, which may be less costly to manufacture. Elastomeric diaphragms may also be desirable in systems where metal diaphragms could chemically react with the process fluid.
Known control diaphragm control valves are designed to use a single type of control diaphragm. For example, known control diaphragm control valves may employ either a metal diaphragm, or an elastomeric diaphragm. Generally, a metal diaphragm cannot be substituted for an elastomeric diaphragm, and vice versa, because the metal and elastomeric diaphragms have different shapes, elasticities, etc., which require different housing configurations.